This kind of knock-type writing instrument includes a knock-type instrument as disclosed in JP Utility Model Registration No. 3139755 by the applicant. This knock-type writing instrument comprises: a shaft tube having a long hole extending axially on its back peripheral wall; a refill having a writing part at its front end and inserted into the shaft tube from the back end of the shaft tube; a return spring biasing the refill backward; a knock member inserted into a back part of the shaft tube to allow its front end to abut the back end of the refill and allow its back part to protrude from the back end of the shaft tube; a resilient piece connected to the front end of the knock member to extend backward and capable of resiliently deforming in a radial direction of the shaft tube; a clip having a clip leg that is connected substantially orthogonally to a surface of the resilient piece and is slidably fitted in the long hole, the clip having a front part compressed into contact with or approaching an outer circumferential surface of the shaft tube; and a lock protrusion formed at the back end of the resilient piece and having a front surface inclined toward an interior of the shaft tube in a direction of the front end of the shaft tube.
In a knock-type writing instrument such as JP 3139755, a lock hole connected to the back end of the long hole via a middle slit as well as a back slit extending from the back end of the lock hole to the back end of the shaft tube are provided on the back peripheral wall of the shaft tube. The clip leg is fitted in the long hole via the lock hole and middle slit from the back slit. When the writing part of the refill is withdrawn within the shaft tube by a bias force of the return spring, the lock protrusion of the resilient piece is locked to the lock hole to maintain this withdrawn state of the writing part. When the back end of the knock member is knocked in such a withdrawn state of the writing part, the clip integral with the knock member and the refill advance, at which point the inclined front surface of the lock protrusion slidingly comes into contact with the inner wall surface of the middle slit, and the resilient piece resiliently deforms inwardly in the radial direction of the shaft tube. This causes the lock protrusion to pass through the middle slit to the long hole. When the lock protrusion moves to the long hole in this way, a resilient restoring force of the resilient piece causes the lock protrusion to be fitted in the long hole to lock the back end of the lock protrusion to the back end wall of the long hole. Thus, the writing part at the front end of the refill is maintained in a writable state in which it protrudes from the front end of the shaft tube.
In this writable state, when a surface of the clip is pressed (knocked), the resilient piece resiliently deforms inwardly in the radial direction of the shaft tube to release the lock of the back end of the lock protrusion to the back end of the long hole, and the refill and knock member are retracted by the bias force of the return spring. Then, when the lock protrusion moves to the lock hole, the resilient restoring force of the resilient piece causes the lock protrusion to fit in the lock hole to lock the back end of the lock protrusion to the back end of the lock hole, thereby maintaining the writing part in the withdrawn state within the shaft tube.
A knock-type writing instrument, such as JP 3139755, is intended to stably keep the writing part protruding from the front end of the shaft tube while increasing a resilient force of the clip, without increasing the number of components. In particular, the knock-type writing instrument of JP 3139755 is characterized in that, in order to stably keep the writing part protruding from the front end of the shaft tube, the lock protrusion, which is locked to the back end of the long hole while the writing part protrudes from the front end of the shaft tube, is formed at the back end of the resilient piece outwardly in the radial direction of the shaft tube. The long hole of the shaft tube is formed into a plane rectangular shape. The clip leg is formed to have a rectangular cross sectional shape as a surface shape in a direction in which its front and back ends are orthogonal to the shaft tube. With the writing part being withdrawn within the shaft tube, the bias force of the return spring causes the back end face of the clip leg to abut the back end face of the long hole in a surface contact state, such that the clip leg does not easily exit the long hole outwardly toward the back end of the shaft tube.